Apparatus for avoiding digester corrosion



Aug. 30, 1960 F. H. RICHTER ETAL 2,950,757

APPARATUS FOR AVOIDING DIGESTER CORROSION Filed Jan. 30, 1953 2 Sheets-Sheet 1 INVENTORS r spiz/ck A. P/GWE? FFEDfP/CK 5. SN van? BY 5' ATTORNEY if 5 5 a! Aug. 30, 1960 F. H. RICHTER ETAL APPARATUS FOR AVOIDING DIGESTER CORROSION Filed Jan. 30, 1953 2 Sheets-Sheet 2 INVENTORS FEEDER/(K A /?/CHTE/? FPEDER/CK B- SNS DEI? ATTO R N EY United States Patent APPARATUS FOR AVOIDING DIGESTER CORROSION Frederick H. Richter, Glen Cove, N.Y., and Frederick B. Snyder, Alliance, Ohio, assignors to The Babeock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Jan. 30, 1953, Ser. No. 334,133

11 Claims. (Cl. 162-233) The present invention relates to the digestion of cellulosic materials in alkaline cooking liquors, and more particularly to methods of and apparatus for retarding the corrosion of the inner face of a digester shell during one phase of the operating cycle.

In the pulp and paper industry wood chips are treated in digesters with a cooking liquor under controlled conditions of temperature, pressure, and time. The cooking liquid in the sulphate process of pulp production is predominantly composed of an aqueous solution of sodium hydroxide and sodium sulfide, and under digestion conditions dissolves the inter-fiber bonding materials of the wood whereby the fibrous material may be subsequently separated from the bonding materials which are in solution in the residual black liquor.

During recent years it has been observed that the corrosion rate of the inner face of the metal walls of the digester shells has been greatly accelerated in the sulphate digesting processes although when sulphides are absent, as in the soda digesting process, the corrosion problem has not experienced a corresponding change. crease in corrosion has reduced the life of the digesters and greatly increased that portion of the cost chargeable to this element of the plant in the production of pump and paper. The reason for the accelerated corrosion is unknown, but has occurred'universally and has adversely afiected the useful life of mild carbon steel, as well as alloyed steel, digester shells. Many attempts have been made to select a suitable alloy for installation in digesters to reduce or eliminate corrosion. Generally speaking, these attempts have not been successful, in that the increased life of the alloy metal has not justified the increased cost of the alloys.

One of the primary causes for the corrosion of digester shells is a direct or indirect result of the concentration of the liquor on the shell of a digester. Under operating procedures sufficient heat is stored in the shell of the digester to cause evaporation and concentration of cooking liquor contacting the digester shell in small quantities. Small quantities of liquor usually contact the hot walls of the digester as a result of splashing of the liquor stream during recharging of the digester and are concentrated by evaporation and apparently cause corrosive attack of the shell metal.

In accordance with the present invention we provide an inner liner spaced from the digester shell which is constructed and arranged for the accumulation of an insulating layer of pulp fibers between the liner and the shell. The heat barrier so formed restricts heat flow from the shell of the digester so that localized evaporation and concentration of incoming cooking liquor is avoided. The liner is constructed of relatively thin perforated plates or screens of low heat storage capacity which are spaced and supported from the inner surface of the digester shell. The liner is constructed for ease of installation, inspection and removal, and is so supported by the digester shell that relative movement between liner and shell, as caused by temperature difference, is accomplished without stressing the assembly.

The in-' ice The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described an embodiment of our invention.

Of the drawings:

Fig. 1 is an elevation view of a pulp digester incorporating the liner of the present invention;

Fig. 2 is an enlarged view of a perforated panel liner section;

Fig. 3 is an end view of the panel liner taken on line 33 of Fig. 2;

Fig. 4 is an enlarged view of a portion of the panel liner and its supporting means shown in Fig. 3;

Fig. 5 is an enlarged plan view of the panel liner supporting means;

Fig. 6 is an exploded perspective view of the panel liner supporting means;

Fig. 7 is a section view taken on the line 77 of Fig. 5;

Fig. 8 is a section view taken on the line 8-8 of Fig. 2;

Fig. 9 is a section view taken on the line 9-:9 of Fig. 8;

Fig. 10 is a plan view of a screen liner;

' Fig. 11 is a section view taken on the line '1l11 of Fig. 10; and

Fig. 12 is an elevation, in section of the upper portion of the digester shown in Fig. 1 arranged for the introduction of wood chips and cooking liquor.

While we illustrate one common type of stationary pulp digester, it will be understood that other types of digesters, such as rotary digesters, are known and in use in this country and abroad. The present invention is applicable to all types of pulp digesters, and for purposes of illustration the liner shown is typical of the construction involved.

As shown in Fig. l, the shell 10 of the digester is of circular cross-section having a domed top 11 and an inverted frusto-conical bottom portion 12. The dome 11 is provided with a central upper opening for the admission of wood chips and cooking liquor. As shown in Fig. 12, a hopper shaped charging spout 19 is inserted into the central opening 13 of the digester for the intra duction of the wood chips, while the liquor enters through a nozzle 29 projecting through one Wall of the spout. The opening is provided with a cap member 14 which fits over the neck of the opening in pressure tight relationship therewith. In the embodiment shown, cooking temperatures and pressure within the digester are attained by the introduction of steam through a pipe 15 and a steam distributor coil 16 which is disposed in the lower portion of the digester shell. The flow of steam to the digester is regulated by manipulation of the valves 17 and 18 in the pipe 15. The digester is provided with a pressure relief connection 20 located in the domed top 11, which is used to reduce the pressure in the digester when the coo is completed. The bottom portion 12 is prodesirableto adda small'amounfof residual black liquor from a previous cooking'cyele'tothenevvcook? The" digester body is filled with wood chips and liquor to a level spaced .below the top 11,, and when this level-.is

: In somesystems ofdi'gester-ropefationithewoodchips' may be'delivered to the .digester beforethe liqiiofisl'in tro'clilcedg 'Othe'r units are operated"witlia"sfifallportion ofthe liquor charge beingfirsfinfrodueed afid'thereafter the wood chips andliquor are simultaneously?introduced until the digester is filled. I

The delivery of. steam to the.filled"digester' displaces maybe trappedin thesubi'n'ergedchip sor: present in the pores"ofthe woodichipsj as wellasthat' whichis presentwithin the 'shellofithe'digester abo've theliquorlevel. During the displacement of therair' 'b-y steamcertain gases such as tur'pentinemnd'acetoneare. liberated, with both the gas. and the air" discharged throughthe vent'connection 20'.

It is common 'fpractice tobring thepressure' within the dfgester" up" to- 120 to" 130 psi. and a temperature of from 340 to 350 F. in a periodot from'l to'2 hours.

After the desire'd'temperature and pressure conditions have. been obtained the steam flowisreduced 'toa'value su'lfieient tocompensate for radiation'los'ses, and the di.-

gester is maintained at the. desired temperature vand pressure"condition for a' cooking period of from "1 to 3 hours.-

As illustrated in the drawingnthe digester is arranged for the direct introduction'of steani'into' the digester by. means of the distributing coil.16. It will be understood" that. in some cases cooking pressure andtemperatureis obta'ined'by the circulation 'of 'liquor'froni' the 'digest'er through a heat exchanger wherein it is heated-"hy steam and. returned to the di'gester.

Aftenmaintainingthe. digester at thelse'lected"temper'-' ature and pressure for thedesiredperiod oftime} the steam supply through pipe 15is cut off and pressure-is reduced-by exhausting steam through the vent connection 20.. Thereafter the discharge 'pipe 21 .isfopened'and'the contentsiofthe digester discharged." This procedure ordi narily takesfrom to minute's andiminediately. thereafter the digester is available for recharging with of "a digester shell may corrode'at a hig'h rate while other area's in the samevessel may'shoW an inconsequential corrosion rate,orn'o corrosion at all." This condition also occurs generally regardless. of"-tl1e composition .of. metal used in constructing the digester. shell. Generally, th'e'shell areas showing no-corrosion ora-lovv rate.of cor-.

rosion are found to be covered 'With'a scaleconsisting.

of calcium carbonate;

We have found'that one of the primary: causes. ofidigester corrosion is apparently the direct-"or. indirectlea sult of a localized increase in the'concentrationof the 'cooking liquor by evaporation from" contact between.

relatively small amounts of cooking.liquoLandfthehot shell'of. the digester. The temperature in-theldigester. shellimmediately following discharge of pulp therefrom will. ordinarily approximate that of. the cooking: temperature,fwhich is of. the order of. 340- F1 Duringthe rechargingofthe digester .the entering. cooking liquor has "a -tendency' to splash :upon,'or.to.otherwise.cor1tact, the digester shell in small amounts. Under. such circumstances the cooking liquoris heated andiapo-rtion thereof vaporized bythe heat stored in thedigestershell, with'a resulting concentration of the liquor.

It will be noted that ordinarily a'digesterwilllbe from 4010250 feet long, and l0 to 15 feet'in diameter With theseidimenSiQn'S and the customary. operatingpressure. utiliz'edin the'digestionprocess," the; digester shellll-will be approximately 1 and A" hick. This thicknessof the shell provides an appreciable heat storage capacity; so" that the shell 10 does not cool very rapidly by heat exchange to the atmosphere.

The excessive corrosion upon the heavy shell can be avoided by the installatiorr ofa" shield or liner spaced from thein nersurfaceiof-thesdigester.v ,As'sh'own in- Figs. 2 to 9, the liner'is disclosedzinitherform of a multiplicity of perforated plates, while in Figs. 10 and 11, the liner. is rdisclosedin'therform ofi a'wiremesksereenf Ineach embodiment of thetinvention; the liner' is" provided; with spacing means for maintaining the liner'in positional'relationship inwardly of the surface of the digester shell 10. As shownlim higst lto? 4, inclusiveg eaeh'perforated panel 22 is rectangular-and-dimensioned to allow passage of individual plates through access openings in the shell of the digester. The panels are desirably thin, for example /s inchgkfor lowheat storagecapacity'"andeconorni'cal-installation. For-mounting purposes;the"corners therthroughwhich-may be oftheform'shownIin'Fi'gs; 2,

8, and- The-number'ofthe 'openingst'may be. 'oflany convenient number and-size; depending'iupon' the dimensin-of the individualplates 225 1 Inthe"example shown,

the panels 22 are approximately. '12by 18 inches;'and the openings 24are'approXimately'2 inches wide. The openings are made'b'y slitting' thepanel; and displacing; the metal in one direction orr'onesi'de of the slit, while the metal is displaced'inthe' opposite "direction on the other side of theslit: With this'construction each of the openings 24 1ies'in"a"plane' generally normal'to the surface of the panel, 'and'is bounded by the edgesof'the plate portions oppositely displaced. on'the sides"ofthe slit.'

The displaced portions' of" the plate form" hoodsrzs and' 26 in'thepanel; n

The panels *22 'are mounted in the digester so that'the h'ood 2'5 of= eachof'the. openings 24 is'up ermost and projectsfinwardly.toward the aXis of the'digester while.

thehood 26pfeach opening is lowermost" andpr'ojects.

toward the shell10: Thus, the hood 26' extends partially into the space '27 between. the panels 22 0f the. linerand the shell10'of the digester'. With-this constructionand arrangement the introduction ofthefwoodlchips and liquofinto "the 'digestenwill'" not cause gravitymovement of "chips into the space between theliner. and the. shell; However, as the level of .cookinghquor andwood chipsrise's' in the digester some liquorand chipswill pass. through the openings 24 into. and accumulate withintthe space 27'between the liner and the shell. 7

The individualpanels 22 of the-'liner are maintained inipo'sition' by T bars"'30"which are spot welded "to they shell'llY'and are circumferentially spaced inaxially extending'parallel "rows so that the edges and flanges 23- of the panels areheld bythecross-bars 31 of-thelT-bars. In'th'e embodiment of the -inventionillustrated, thet-pam els 22"are installedin longitudinal rows in the'digester withradjacent panelsin each row abutting; each other. Each" row of panels. 22 is positioned longitudinally inthe. digester by upper and lower cap -members32which are. spot: welded. to the 'T-bars. Thespacing between adjacent bars 30 is such thatthe panels-22 fit loosely therebetween so that difierentialexpansion for: the liner and' the shell will neitherbind nor permit thepanels to fall-out of positionduring operation of.theudigester;

The, T-bars; 30 are 1 constructed.-inwsectionsi with: a tongue and" groove joint therebetween, illustrated in Figs.'.-2, -5; 6, and 7. The-groove 33"inoneendaof a cross-hat 31 n gagesia matchingztongueii l on'itlieieml.

of an adjacent T-bar 30. Referring particularly to Fig. 6, it will be noted the leg 35 of the end of one T-bar is cut back so that the grooved cross-bar 31 extends beyond the end of the leg while the leg 35 of the matching end of the adjacent T-bar is extended beyond the tongue 34. As shown, the leg 35 of the T-bar 30 is spot welded to the shell it? at one end portion thereof, as at 36, so that the opposite end of each T-bar section is free to move longitudinally in the tongue and groove joint in accordance with temperature changes occurring during operation of the digester.

The liner shown in Figs. and 11 includes a screen formed from comparatively light gauge wire 37 having,

for example, /1 inch openings 38. The screen may be of one piece construction to line the shell of the digester or may be made in sections with adjoining sections installed in abutting relationship internally of the digester shell. The size of screen section is determined by the size of the access openings in the digester shell.

The screen is mounted in a spaced position inwardly of the shell 10 by means of circumferentially and longitudinally spaced supporting means. In the embodiment, the supporting means consists of a length of pipe 40 which is Welded at one end to the inner surface of the shell 10 and projects in a general radial direction toward the longitudinal axis of the digester. A collar is constructed from a piece of pipe 41 having an internal diameter greater than the external diameter of, and having a lesser length than, the pipe 40. A washer 42 having a central opening 43 therethrough of a diameter greater than the external diameter of the pipe 40 is welded to one end of the pipe 41. The assembly of pipe 41 and washer 42 is inserted over the pipe 40, with the Washer 42 spaced from the shell 10. The Wires 37 of the screen are displaced as necessary to fit over the pipe 4% and the screen held against the washer 42 by a second washer 44 which is held in place by a rod member 45 which is inserted through a hole 46 drilled through the end portion of the pipe 40. The rod member 45 is bent to maintain its holding position in the pipe 40. With the structure described the screen is maintained in a spaced position inwardly of the digester shell 19. The circumferential and longitudinal spacing of the pipes 49 is sufficient to maintain the screen liner away from the shell It) to provide a space 27 therebetween similar to the space 27 between the panels 22 and shell 19 shown in Figs. 9 and 10.

In the operation of a digester equipped with the liners described, a mat of pulp fibers is trapped in the space 27 (or 27) which serves as an insulating blanket which not only prevents splashing liquor from directly contact ing the shell 10 of the digester but also permits the liner to release heat at the end of a cooling cycle so that the heat storage therein is not sufficient to cause the surface boiling and corrosion conditions described. At the end of the pulp cooking period the pulp fibers in the space 2.7 are saturated with liquor, and when the digester is pressure relieved during blow-down some of the liquor in the fibers flashes to a vapor by the pressure reduction. The stored heat in the relatively thin liner is transferred to the liquor discharged from the digester, resulting in lowering the temperature of the liner to a temperature of the order of 220 F. The fibers accumulated in space 27 also serve as an insulation to prevent regain of heat in the liner from the shell during the recharging of the digester. With the substantial elimination of surface boiling conditions, corrosion resulting from localized concentrations of cooking liquor is greatly retarded, if not avoided. We have found that the liner, whether in the form of perforated panels or a screen, can be constructed of mild steel without serious corrosion difficulties, even though corrosion of alloy metal shells had occurred in the same digester prior to the installation of the liners of the present invention.

As disclosed and claimed in copending applications of R. H. Richter digester corrosion can be retarded by 0001-" ing the shell of the digester to a temperature of the order of 220 F. by heat exchange with a heat transfer medium, Serial No. 334,132, filed January 30, 1953; and by the introduction of the cooking liquor into the lower end portion of the digester so as to form a quiescent pool with -a gradually increasing level until the digester has been.

filled, Serial No. 334,134, filed January 30, 1953. In both applications localized concentration of the liquor by evaporation from surface or hot face boiling is avoided.

While in accordance with the provisions of the statutes we have illustrated and described herein the best forms and mode of operation of the invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a digester, metallic walls defining an elongated body having valved inlet and outlet openings therein for the admission of Wood chips and cooking liquor and for the discharge of pulp, and means for accumulating a layer of heat insulating pulp fibers adjacent the inner surface of said metallic walls comprising metallic liner means inwardly spaced from said digester walls, said liner means including rows of perforated panels having flanged edges, and T-bar spacer means attached to the walls of said digester and overlaying the edges of said panels.

2. In a digester, metallic walls defining an elongated body of circular section having valved inlet and outlet openings therein for the admission of wood chips and cooking liquor and for the discharge of pulp, and means for accumulating a layer of heat insulating pulp fibers adjacent the inner surface of said metallic walls comprising an inner metallic liner means having flanged edges spaced from said digester walls, said liner means including rows of perforated panels, and rows of circumferentially spaced T-bars extending longitudinally of the walls of said digester for positioning said panels.

3. In a digester, metallic walls defining a pressure vessel having valved inlet and outlet openings therein for the admission of cellulosic material and cooking liquor and for the. discharge of pulp, and means for accumulating a layer of heat insulating pulp fibers adjacent the inner surface of said metallic walls comprising liner means positioned internally of said vessel walls comprising rows of perforated panels having edges turned toward said walls, and T-bar members spot welded to the inner surface of said walls and arranged in overlaying relationship to the edge surfaces of said rows of panels.

4. In a digester, metallic walls defining an upright cylindrical pressure vessel having valved inlet and outlet openings therein for the admission of wood chips and cooking liquor thereto and for the discharge of pulp therefrom, and means for accumulating a layer of heat insulating pulp fibers adjacent the inner surface of said metallic walls comprising metallic panels spaced inwardly of said digester walls of said pressure vessel, said panels having upwardly opening hooded openings therethrough for the movement of pulp therethrough into the space between said panels and said pressure vessel walls.

5. In a digester, metallic walls defining a pressure vessel having valved inlet and outlet openings therein for the admission of cellulosic material and cooking liquor thereto and for the discharge of pulp therefrom, means for accumulating a layer of heat insulating pulp fibers adjacent the inner surface of said metallic Walls comprising liner means spaced from the internal surface of said digester 'walls, said liner means comprising a plurality of perforated panels having flanged edges, and spacing means for said panels comprising parallel rows of T-bars, each row formed in sections with one end portion of each section welded to the inner surface of said digester waibandf connected: with an sadjoiningi-wsectiombyg-a :tofigue? t fiz-t kdigesten for the- :pr'e'paration:ofi pulp by the"batch--- digestion tof celIu'l'osic fmaterials' in --a cooking; liquor con1-' prisingtsmetallic walls havingra relatively high heat storage capacitycdefiningaazdigester body, meansvformingqa valved i inletr andi a: valved outlet in said metallic -digester wallsfor the. admission r of cellulosie materials and'l cookin'g-z liquorfto andtfor theedischa-rge: of pulpvfrom said idigester means for accumulating a layer: ofi-= heat.- insulating,

pulpryfibers'i adjacent the inner surface'of said' digest'er walls'fcomprisingavlinerspaced andsupported; from fthe;

and means for accumulating alayer of heat insulatingi pulpwfiberss adjacent theinnersurface of said-"digester wall scomprisingaa metallieliner of relatively low heat storagescapacity having openings therein -.ofsufiicientsize for movementof cellulosic materialsand cookingi liquor therethrough, and a-pluralityiof-tlaterally spaced-supports for locating said liner'at aspacedp osition internally of said digester 'walls.

' 8;"A digester as defined in-claim-Gwherein saidiliner comprises a plurality of individually supported perforated panel s.-

comprises a wiretmesh screen:

10: A di'gester-astdcfined'in claim'6 wherein-said linercomprises -a: wiremesh screen, a-pl-urality of-longitudinally and circumferentiallw spaced members attached topand projeeting inwardlyyof thewalls of said pressure vessel, a first washer associatedt with said-member andispaced-v and circumferentially spaced-t members welded to the inner surface of. said walls and-projecting inwardly of said pressure vessel, a first; washer associatedtwith said member and spaced: fromsaid wallito position said screen, i

and a 'seeond washerassociated with saidmember and positioned-to retain said screen on said first washer.

References Citedin the file of patent 'I INI TEDi sTATEs PATENTS Re;- 4, 771 Sinclair Feb. 27, 1872 106,135 Dean 1 Aug; 9, .1870 41111838 Springer; .Oct. 1, 1889 620,243 Lyman Feb; 28, 1899" I 638,267- Norton Dec; 5, 1899i- 881388 Atwood t Mar. 17, 19081 889,158 Spencer: Nov; 29; 1932 1,91 1,608 Daviset al May-30, 1933' 2,614,923 Tarkkonen Oct; 21, 1952:

FOREIGN PATENTS Great Britain 7 V of 1883 

